1. Field of the Invention
The present invention relates to an insulating film used as a tunnel oxide film of a flash memory, a gate oxide film of a transistor or the like, and to a method of forming such a film.
2. Description of the Related Art
Apart from the memory for general use, such as DRAM, SRAM or the like, recently there has been an increasing demand of EEPROM (flash memory), in which data remains if the power is turned off, data can be rewritten for 10.sup.6 times or more, and which is suitable for a large increase in capacity. The reliably of such a flash memory is deeply correlated with the reliability of the silicon oxide film (insulating film) having a thickness of 10 nm or less, which is called a tunnel oxide film. Thus, the reliability of the flash memory is greatly influenced by three different electrical characteristics: (1) dielectric breakdown life, (2) charge trap amount and (3) stress leak amount, which are items of evaluating the reliability of a silicon oxide film. If at least one of these characteristics is deteriorated, the device cannot function sufficiently.
As described above, it is necessary for a flash memory to keep up the above three reliabilities at the same time; however, with a conventional thermal oxide film, all of the reliabilities cannot be satisfied. For example, as to the conventional thermal oxide film, there has been a report of achieving the prolongation of the dielectric breakdown life and the reduction of the charge trap amount; however, there has been no report of achieving a significant reduction in the stress leak amount.
Further, in the case of the gate oxide film of a transistor, for example, the variance of the element characteristics occurs due to hot carrier implantation. The hot carrier implantation is a phenomenon in which electrons in a channel of a transistor becomes hot as energy being supplied from an electric field in the direction along the channel, and the electrons are implanted into the gate oxide film. This phenomenon generates a charge trap in an oxide film, or an interface state at an interface between the oxide film and the substrates, which causes a variation in the threshold value of the transistor or a gm deterioration.
Meanwhile, with regard to the method of forming a tunnel oxide film, in the case where an oxide film having a thickness of about 5 to 10 nm is formed by a conventional vertical diffusion furnace, it has been reported that an oxied film, which was oxidized in a water vapor atmosphere, in a so-called wet oxidizing atmosphere, has an insulation breakage life longer than an oxide film, which was oxidized at a temperature of 850.degree., in a so-called dry oxidizing atmosphere (dry oxidizing atmosphere). Further, with regard to the amount of stress leak generated, it has been reported that the amount of the stress leak of an oxide film formed in a water vapor oxidizing atmosphere is suppressed more (lower) than the case of an oxide film formed in a dry oxygen atmosphere. Meanwhile, it has been reported that the density of the charge trap is reduced more in an oxide film formed in a dry oxygen atmosphere than in an oxide film formed in a water vapor atmosphere. However, there has been no report on a silicon thermal oxide film or a method of forming such a film, which satisfies all of the above-described three reliabilities at the same time.
As described, with a silicon thermal oxide film, by itself, it is not possible to satisfy the three reliabilities at the same time, and therefore a silicon oxynitride film in which nitrogen is introduced into a silicon thermal oxide film, is presently used as a tunnel oxide film. The silicon oxynitride film is able to decrease the stress leak amount, and further satisfies the conditions of a dielectric breakage life which the silicon thermal oxide film itself possesses and a decrease in the charge trap amount. Thus, the silicon oxynitride film satisfies the above-described reliabilities at the same time.
However, the silicon oxynitride film has current-voltage (I-V) characteristics different from those of a silicon oxide film which does not contain nitrogen, since nitrogen is introduced to a silicon thermal oxide film. Also, the device operation characteristics are varied along with the amount of nitrogen introduced, which creates a problem. As typically exemplified in the thinning of a thermal oxide film at an element separation edge, the three reliabilities are very much influenced by the reliability of the silicon thermal oxide film itself, which serves as an underlayer. In reality, it is necessary to improve the film quality of the silicon thermal oxide film must be further improved in terms of all of the three reliabilities, from those of the present status.